NFX|高压储氢复合材料储罐分析及厚度优化
ABSTRACT:
This study presents a comprehensive approach to the design and analysis of a compressed hydrogen tank which, aims in optimizing both safety and performance parameters. The design phase involves the exploration of various materials and structural configurations to determine an optimal combination that meets industry standards and regulatory requirements. Special attention is given to lightweight, high-strength materials that can withstand the demanding conditions of compressed hydrogen storage. Finite element analysis (FEA) is employed to simulate and assess the structural behaviour of the tank under different loading scenarios, including internal pressure, external forces and thermal effects. The results of the finite element analysis are used to refine the design, optimizing the tank's geometry and material selection to enhance both safety and efficiency. The findings contribute valuable insights to the ongoing efforts in developing hydrogen storage systems that are not only technically robust but also economically viable. The outcomes of this study have the potential to inform industry practices and regulatory standards, fostering the advancement of sustainable energy technologies.
KEYWORDS:
Clean energy; Composite tank; Finite element analyais; Compressed hydrogen tank
1.Introduction
A composite tank for hydrogen storage represents an innovative and efficient solution in the realm of hydrogen storage technology. Unlike traditional metal tanks, composite tanks are constructed using advanced materials such as carbon fiber reinforced polymers, which offer a unique combination of strength, lightness and corrosion resistance. This results in a tank that is not only significantly lighter than its metal counterparts but also provides enhanced durability and safety. The use of composite materials allows for higher pressure storage, maximizing the volumetric efficiency of hydrogen storage systems. These tanks play a crucial role in the development of hydrogen-powered vehicles and renewable energy applications, offering a lightweight and durable means of storing and transporting hydrogen, contributing to the ongoing efforts to establish hydrogen as a clean and sustainable energy carrier for the future where different types of hydrogen storage tanks are based on their construction and materials. These classifications are primarily used in the context of high-pressure hydrogen storage for fuel cell vehicles [3]. Type I hydrogen tanks are made of steel. They are all-metal tanks and rely solely on the strength of the material to contain high-pressure hydrogen. Type I tanks are generally heavier compared to other types because of the thickness and weight of the steel. Type II hydrogen tanks have a thin inner liner made of a lightweight material(such as aluminum) and are reinforced with a composite material, usually carbon fiber. The composite material provides additional strength while reducing weight. Type II tanks are lighter than Type I tanks, making them more suitable for automotive applications.
Type III hydrogen tanks are made up of a composite structure, with a thin liner made of a lightweight material (e.g., aluminum) reinforced with a composite material (carbon fiber or fiberglass). The composite layers provide structural strength and reduce the overall weight of the tank. Type III tanks are lighter than both Type I and Type II tanks. Type IV hydrogen tanks feature a polymer liner (such as high-density polyethylene) with a carbon fiber composite overwrap. The use of a lightweight polymer liner, along with the composite overwrap, makes Type IV tanks lighter than other types. Type IV tanks are known for their low weight and high strength. Type V hydrogen tanks typically involve advanced composite materials, often incorporating innovative designs and manufacturing processes. These tanks are designed to be even lighter and more efficient than Type IV tanks, often utilizing advanced materials to achieve optimal strength-to-weight ratios. Type V tanks aim to further reduce weight while maintaining high structural integrity. The classification into types helps describe the evolution of hydrogen storage tank technology, with each type representing advancements in materials and design to improve efficiency, weight and safety. The choice of tank type depends on factors such as weight considerations, safety requirements and intended applications in industries like transportation [4]. Fig. 1 shows the different types of the hydrogen tanks. Table 1 describes the materials and applications of hydrogen tanks.
The Type IV composite tank for hydrogen represents a cutting-edge advancement in the field of hydrogen storage technology. This tank design, classified as "Type IV" according to industry standards, incorporates a multi-layered structure where the hydrogen is stored in a plastic liner reinforced with carbon fiber or other high-strength materials [2]. The outer layer of the tank is typically made of a non-metallic material, such as fiberglass, providing an additional layer of protection against external factors and enhancing overall safety. Type IV tanks are renowned for their lightweight nature, making them particularly well-suited for automotive applications. The innovative design of the Type IV composite tank ensures a high level of safety, as the plastic liner effectively contains the hydrogen, while the outer layers contribute to the structural integrity of the tank. This technology plays a pivotal role in promoting the widespread adoption of hydrogen fuel cell vehicles and contributes to the development of a cleaner and more sustainable transportation infrastructure. A liner is typically used as an inner layer within the vessel to contain and store the hydrogen gas. The purpose of the liner is to provide a barrier between the high-pressure hydrogen and the outer shell of the pressure vessel. Hydrogen, being a small and light molecule, can permeate through certain materials over time. To prevent the loss of hydrogen and to maintain the integrity of the pressure vessel, a liner made of materials with good hydrogen barrier properties is employed [8]. The liner acts as a barrier to minimize permeation and maintain the structural integrity of the pressure vessel. Common materials used for liners in hydrogen storage vessels include high-strength metals, polymers, or composite materials that are designed to withstand the high pressures associated with hydrogen storage. The choice of liner material depends on factors such as the required strength, weight considerations and the ability to prevent hydrogen permeation. It's important for the liner to be compatible with hydrogen to avoid embrittlement or other degradation issues over time. Additionally, the design and manufacturing of pressure vessels for hydrogen storage must adhere to safety standards and regulations to ensure the reliable and safe storage of hydrogen [6].
2.Design requirements
Designing a liquid hydrogen (LH2) tank involves meeting specific requirements to ensure safety, efficiency and durability. The design considerations may vary depending on the application, whether it is for storage, transportation, or use in a specific system. Materials used in the construction of LH2 tanks must have low-temperature compatibility, as hydrogen exists in a cryogenic state at very low temperatures. Common materials include stainless steel, aluminium alloys, or composite materials such as carbon fibre-reinforced polymers. Effective thermal insulation is crucial to minimize the heat transfer and maintain the low temperatures required for storing liquid hydrogen. Insulation methods may include vacuum insulation, multilayer insulation, or foam insulation. The tank structure must be designed to withstand the internal pressure of the liquid hydrogen and any external loads during transportation or handling. Proper stress analysis, fatigue assessment and consideration of thermal stresses are essential for ensuring structural integrity.
Safety is a top priority and the tank design should incorporate features to prevent leaks, control pressure and allow for emergency venting if necessary. Pressure relief devices, rupture disks and other safety mechanisms are often integrated into the design. The tank must be designed to handle the pressure associated with storing or transporting liquid hydrogen. The pressure rating is determined by factors such as the temperature of the stored hydrogen. Preventing hydrogen permeation through tank materials is crucial to minimize losses and maintain the integrity of the stored hydrogen [10]. Material compatibility with hydrogen is a key considerationFor applications like transportation, reducing the weight of the LH2 tank is essential to improve fuel efficiency and overall performance. Composite materials, such as carbon fibre-reinforced polymers, are often used to achieve a balance between strength and weight. The design must comply with relevant safety standards, codes and regulations for hydrogen storage, transportation and usage. Efficient and safe systems for filling and draining liquid hydrogen from the tank should be designed to prevent overfilling, ensure proper venting and facilitate easy handling. Implementing sensors and monitoring systems is crucial for real-time tracking of temperature, pressure and other critical parameters to ensure safe operation and maintenance. Tanks may be subjected to various mechanical stresses during transportation and handling.
Design considerations should account for impact resistance to prevent damage to the tank. Proper seals and gaskets are necessary to maintain the integrity of the tank and prevent hydrogen leakage. These design requirements are critical for developing LH2 tanks that meet safety standards, minimize losses and facilitate the efficient storage and transportation of liquid hydrogen. Depending on the specific application, additional considerations may be necessary. Collaboration with relevant regulatory authorities and industry standards organizations is also essential in ensuring compliance and safety. The general considerations for pressure and temperature in a Type IV hydrogen tank are, Type IV tanks are designed to operate at high pressures. Common pressure ratings for Type IV tanks range from approximately 350 to 700 bar (5,000 to 10,000 psi). The specific pressure can vary based on the intended application and design specifications. The hydrogen stored in Type IV tanks is subject to ambient temperatures unless specific heating or cooling systems are employed for a particular application. The material choices, such as the polymer liner and the composite overwrap, are selected to maintain the structural integrity of the tank at both ambient and operational temperatures. It's important to note that the pressure and temperature conditions can vary during the filling and emptying of the tank, as well as under different operating conditions. The design of Type IV tanks considers factors such as safety, weight reduction and durability [1].
3.Material requirement
Liquid hydrogen (LH2) is stored and transported in specialized tanks designed to handle the extreme temperatures and low-pressure conditions associated with this cryogenic fluid. The materials used in making liquid hydrogen tanks need to possess properties that ensure safety, durability and insulation. Stainless Steel is often used for the inner vessel of the tank. It provides good strength and corrosion resistance. Stainless steel alloys with low thermal conductivity are preferred to minimize heat transfer. Aluminum Alloys are commonly used for the outer vessel of the tank. They are lightweight and have good thermal conductivity. Additionally, aluminum has a low-temperature coefficient of expansion, which is important for cryogenic applications. Some tanks may use nickel alloys, such as Inconel, for their excellent low-temperature properties and resistance to hydrogen embrittlement. Carbon fiber reinforced polymers (CFRP), may be used for tank construction. CFRP tanks can be lightweight and have good strength, making them suitable for aerospace applications. To minimize heat transfer and maintain the low temperatures required for storing liquid hydrogen, tanks are typically equipped with multi-layered insulation. This insulation may include materials like perlite, aerogel, or other low-conductance materials. Polyurethane foam is used as an insulating material in the space between the inner and outer vessels. It helps to reduce heat transfer and maintain the low temperature of the liquid hydrogen. Some specific steels designed for cryogenic applications may be used in tank construction due to their ability to maintain toughness and ductility at extremely low temperatures.
It's important to note that the selection of materials depends on various factors, including the intended use, safety regulations and specific design requirements. Additionally, the design of liquid hydrogen tanks is a complex process that considers factors such as thermal contraction, pressure containment and material compatibility with hydrogen. Safety standards and regulations must be strictly adhered to in the design and manufacturing of liquid hydrogen storage systems. Cryogenic materials are specially designed to maintain their mechanical properties at extremely low temperatures, typically below -150°C (-238°F). These materials are crucial for applications in industries such as aerospace, energy and gas processing where materials are subjected to cryogenic conditions. Cryogenic temperatures pose unique challenges to materials, as they can lead to increased brittleness and reduced toughness. Cryogenic materials are formulated to withstand these challenges and retain their strength and ductility in such harsh environments. Cryogenic steels find applications in industries where low-temperature performance is essential, including the fabrication of cryogenic storage tanks, pipelines for transporting liquefied gases and components for aerospace and energy systems [11].
4.Mathematic modelling
This involves the development and application of mathematical equations to describe the behaviour of the vessel under various conditions. The primary goal of this modelling is to gain insights into the structural, thermal and fluid dynamics aspects of the pressure vessel, facilitating the prediction of its performance. The mathematical model considers factors such as material properties, geometric configuration and operational parameters to simulate the pressure and temperature distribution within the tank. These models aid in optimizing the design for factors like weight reduction, safety and efficiency while ensuring compliance with regulatory standards. Mathematical modelling can simulate a range of scenarios, helping refine and validate the design of hydrogen tank pressure vessels for various applications, from fuel cell vehicles to industrial storage systems.
The major factor considered while designing a cylindrical vessel is the hoop stress. Hoop stress refers to the stress that acts circumferentially (in the hoop direction) in a cylindrical or spherical object subjected to internal or external pressure. It arises due to the tendency of a pressure vessel, such as a pipe or a container, to expand radially when subjected to internal pressure. The formula for calculating hoop stress (
) in a thin-walled pressure vessel is given by:
Where P is the internal pressure, d is the diameter of the vessel, t is the thickness of the wall. Hoop stress is a critical consideration in the design and analysis of pressure vessels to ensure that the material can withstand the internal pressure without failure. These stress calculations helps to determine the appropriate material thickness and design parameters for pressure vessels to meet safety standards and operational requirements. The tank is modelled according to the required dimensions using the inner and outer diameter of the vessel which was calculated from the thickness. The geometry of the vessel is obtained using the guidelines given in ASME section VIII. After deriving dimensions based on the guidelines, a three-dimensional solid CAD (Computer Aided Design) model was designed using Onshape tool. The preliminary model of the vessel is shown in Fig. 2. The model shows the inner liner made of HDPE polymer and the outer wrap made from the Carbon Fibre. The Fig. 2(a) shows the complete 3D model of the vessel and Fig. 2(b) represents the section view to present the inner and outer layers.
5.Finite element analysis
Finite Element Analysis (FEA) using midas NFX software for hydrogen tanks involves a comprehensive and sophisticated approach to assess structural integrity and performance. In Midas NFX, the initial step is to accurately model the geometry of the hydrogen tank, considering its intricate features. Material properties specific to the tank material and its compatibility with hydrogen are then defined. The loading conditions, including internal pressure from stored hydrogen and other external forces, are applied to simulate real-world scenarios. midas NFX facilitates efficient mesh generation, allowing for a fine mesh that captures localized variations in stress and deformation. With precise boundary conditions specifying connections and supports, the solver settings are configured for static, dynamic, or transient analyses based on the tank's operational requirements. The software provides detailed results, including stress distributions, deformations and safety factors, aiding in a comprehensive assessment of the tank's structural behaviour.
midas NFX can be used to optimize the design iteratively, ensuring the hydrogen tank meets safety standards and regulatory compliance while optimizing for weight, cost and performance. The tool's validation features help confirm the accuracy of simulation results by comparing them with experimental data or analytical solutions, providing confidence in the integrity of the hydrogen tank design. The geometry is split into two halves to observe the impact of pressure acting on the inner as well as outer walls of the tank. The stress acting on type IV hydrogen tank is observed using Midas NFX 2023 R1 software. The hydrogen tank is meshed with a size of 6mm along the inner and outer liners as shown in Fig. 3. The material properties of inner liner terms to be HDPE blow moulding grade properties and the external winding is provided with carbon fibre prepreg properties. The thickness of inner as well as outer liner is assigned individually in property column. The carbon fibre filament wound outer liner is modelled using stacking sequence [-30/+30/-85/+85].
The inner liner being HDPE is assumed to have around 10 mm thickness whereas the outer region is filament winded using carbon fiber material. The thickness of CFRP winding is optimized by providing various thickness inputs to observe the stress range acting on the tank. Contacts are assigned to inner as well as outer liners to indicate that they are bound together. The inner liner is assigned as master and outer liner acts as slave with respect to inner liner [5, 7]. The top and bottom as well as walls of the inner liner depicts fixed region. The fixed boundary condition on wall determines the datum line and the zone at which fluid medium is stored as shown in Fig. 4. Similarly, the fixed condition of outer liner is assigned. The pressure of 70 MPa is given as input in the inner region of tank and the stress and displacement is measured on the outer wall of the vessel. A series of thickness was iterated for the model to understand the displacement and stress variation across the vessel [9]. The series of thickness is shown in the Table 2.
A series of analysis was conducted in midas NFX for the thickness optimisation of the vessel. It is observed that the increase in thickness had impacts in the deformation and stress on the vessel walls. The tank is filament winded using the following sequence [-30/+30/-85/+85] to obtain greater stability as a helical winding incorporates both axial and hoop strength and thus creating a tank with higher durability. Initially, few loops of hoop winds are provided to avoid slippage and increase stiffness while winding. The deformation and stress variation observed during the thickness variation iteration is given in the below Fig. 5. The hydrogen storage tank is optimized to various thicknesses to observe the behaviour of hydrogen tank. It was observed that the deformation and stress showed similar characteristics. The increase in thickness decreased the deformation of the tank thereby assuring a stabilized build and the stress concentrations on the outer wall of the vessel decreases with increase in thickness which increases duration of the product life. The optimized design wherein modelled in a way that it has uniform thickness over shell and dish ends so the structural integrity of the vessel is increased and can be functional for a long range of service life.
6.Conclusion
In this paper, a detailed review of different types of compressed hydrogen tank configuration, design rules and key materials considerations are initially presented to facility a design. The designed tank was then meshed using midas NFX finite element analysis software and then solved for linear static strength substantiation. The obtained deformation and max. principal stress were iterated for a number of thickness to establish an optimum thickness and winding angle for a given stress level and regulatory compliant margin of safety. Composite applications give a promising lightweight compressed hydrogen storage tank for aircraft propulsion and also ground transportation needs of the industries.
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